The instant invention relates to a process for the drying of a food material especially a proteinaceous material, and specifically to a process for air drying wherein the drying air is heated by direct contact with a burner which is operated in such a manner that the formation of undesirable burner combustion product residues in the proteinaceous material, especially nitrite residues, is substantially reduced.
Dried food materials such as proteinaceous materials are widely used as ingredients in foodstuffs, either for protein supplementation or as an alternative for a proteinaceous ingredient such as fresh milk, or eggs. Materials of this type include milk derived protein such as dried casein, casein and whey mixtures, or egg derived protein such as albumin. Also employed are vegetable protein materials, including isolates or concentrates derived from various vegetable protein materials such as soybeans, cottonseed, peanuts, and the like. Dried isolates obtained from the isolation of the proteinaceous material in vegetable protein sources such as soybeans, are widely used as a food ingredient. These isolates are usually produced by the extraction and solubilization of the proteinaceous component in the vegetable protein material, followed by centrifugation or clarification to remove residual, unextracted materials.
The solubilized proteinaceous material, which is typically extracted from the proteinaceous source in a neutral or alkaline aqueous medium is then acidified and the pH of the extract adjusted to the isoelectric point of the protein. The proteinaceous material precipitates at the isoelectric point and is then concentrated by centrifugation or similar means. The concentrated protein material can then be neutralized if desired, retained at or near the isoelectric point, or further processed to improve the functional properties of the proteinaceous material. Typical processes which illustrate further treatment of the proteinaceous material for deflavoring, or an increase in the functional properties of the protein material are those described in U.S. Pat. Nos. 3,642,490; 3,645,745; 3,830,942; and 3,694,221.
Regardless of the type of further processing involved with a protein isolate, dewatering of the material is conventionally carried out to provide a dried material since the dried material presents no storage problems, and is more convenient to use as a food ingredient. Dewatering of the proteinaceous material can be varied out by a variety of means which are conventional in the art. The most desirable means of dewatering is to employ a stream of heated air which comes in contact with the proteinaceous slurry in order to remove free water as quickly and efficiently as possible without adversely affecting the functional properties of the proteinaceous material.
The most widely used process of this general type is referred to as spray drying. Spray drying generally consists of atomizing the proteinaceous material or slurry into a drying chamber by forcing the product through a nozzle at a high pressure in order to break up the liquid. As the atomized product is introduced into the drying chamber, heated air is also forced through the chamber. The air furnishes heat for evaporation of the moisture and the air becomes the carrier for removal of the moisture from the dryer. The air for drying is typicaly filtered and heated before it passes through the atomized product and can be forced through the chamber by a pressure or suction system.
The air for drying may be heated in a variety of ways including the use of an indirect heater or a direct fired, indirect heater, either of which are operated by burning of the fuel and transferring the heat through a metal surface to the air. A direct fired unit may also be used in which combustion products from the flame or burner enter the dryer since the flame is in direct contact with the air used for drying. The direct fired units have the advantage of being more efficient due to less heat loss during heat transfer and the resultant higher temperature of the inlet air into the chamber for drying. The dried product is separated from the air stream after drying, cooled and packaged.
While a direct fired unit provides a very efficient means of heating the inlet air for a spray drying system, nevertheless, a disadvantage of systems of this type is that combustion products from the flame enter the heated air stream. Typical combustion products from the flame include gases such as nitrogen or carbon oxides. The nitrogen oxides are a particular problem in the drying of food ingredients such as proteinaceous materials since nitrous oxide or nitrogen dioxide produced by combustion are introduced into the air used for drying and because of the presence of water in the product to be dried are converted to nitrous acid (HNO.sub.2) in the product. If the proteinaceous material is still somewhat acidic, the nitrous acid formed in the product is somewhat stable and doesn't readily dissociate or ionize to form nitrite which can be retained in the product. The relative stability of the HNO.sub.2 in the acidic environment makes removal of this material with the water somewhat easier and there is less tendency to form residual levels of nitrite in the product if it is dried at an acidic pH, usually below 6.0. If, however, the environment is not acidic, the HNO.sub.2 tends to ionize or disassociate to result in the formation of nitrite (NO.sub.2.sup.-) which is not readily removed with the water during drying because of its ionic form. This results in the creation of an undesirable residue of nitrite in the dried proteinaceous material. Therefore, a means is needed to achieve satisfactory dewatering or drying of a proteinaceous ingredient, whether at an acidic pH or not, including vegetable protein isolates, with direct flame heating of the air stream used for drying, to reduce the formation of residues of nitrite in the dried proteinaceous ingredient.
It is, therefore, an object of the present invention to provide an improved process for the drying of a proteinaceous material using direct flame heating of the air stream used for moisture removal. It is a specific objective of the present invention to provide a process for the drying of proteinaceous materials with direct flame heating of the air stream for drying wherein the formation of nitrite or other combustion product residues in the dried proteinaceous material is substantially reduced.